In order for bluetooth to switch to USB dongle by default use the following command:

sudo nvram bluetoothHostControllerSwitchBehavior=always

and switch back with

sudo nvram bluetoothHostControllerSwitchBehavior=never

Note that change is applied at next reboot.

In oder to undergo more advanced bluetooth debugging use bluetooth explorer program available in additional tools for xcode after registering for free as an apple developper via https://developer.apple.com/download/more/.

So far I have not been exposed (as of yet) to BT disconnectivity hell.

What can I say: it is sad that Apple has not fixed or recognize this epidemic BT failure happening for their iMAC users.